1. Field of the Invention
The field of the invention relates to an apparatus and to a process for growing crystals of semiconductor materials by using a double crucible which has an inner crucible and an outer crucible and in which the semiconductor material is continually supplied in a space formed between the inner and outer crucible.
2. Prior Art
In a batch-type apparatus for growing silicon single-crystal ingots, the quantity of melted silicon contained in a quartz crucible decreases as the silicon single crystal grows. As a result, qualities such as oxygen concentration, crystallization front, and dopant concentration are not uniform longitudinally within the crystalline ingot obtained by the batch-type apparatus. More specifically, the oxygen concentration in the crystal depends on the quantity of oxygen dissolved in the silicon melt, thereby varying as the silicon melt in the quartz crucible varies. In addition, since the quantity of the melt varies, the thermal distribution in the furnace fluctuates and the convection in the melt varies, so that the crystallization front varies. Furthermore, the dopant, such as phosphorus, boron, antimony, and the like, is generally added to the melt in order to control the electric conductivity of the single crystal. Since the segregation constant of such impurities is not identical to 1, the dopant concentration in the crystal differs as the crystal grows.
Thus, the quality of the single crystal ingot varies in the longitudinal direction, and therefore only a portion of the ingot has the desired qualities. Additionally, productivity has been low in such batch processes.
The approach to resolve the above disadvantages is to use a continually-supplied-type apparatus hitherto proposed wherein the single crystal ingot is grown while the quartz crucible is being charged with material. U.S. Pat. No. 2,892,739 and Japanese Patent Application First Publication "Kokai" No. 61-36197 describe one such apparatus which is the simplest crucible characterized by the combination of a continually-supplied-type crucible charging of the material and a two-wall-type crucible.
The above continually-supplied-type apparatus has, however, the following problems. First, although the melting point of the silicon is above 1420.degree. C., the quartz of which the crucible is composed begins to soften at about 1100.degree. C. As a result, the outer crucible supported by a graphite susceptor is not greatly deformed from its original shape greatly and the inner crucible may be deformed since the inner one is not supported sufficiently, so that the growing of the single crystal is adversely affected. Specifically, a great quantity of heat must be applied to the crucible in the initial stage of the pulling cycle in order to melt the material in the crucible, and the temperature in the furnace reaches the maximum at that time. Therefore, the deformation of the inner crucible is greatest at that time. FIGS. 1 to 4 illustrate conventional apparatuses each of which comprises a double quartz crucible. The double quartz crucible assembly is housed in a graphite susceptor 103 and comprises an inner crucible 101 and an outer crucible 102, and a resistance heater 104. The double crucible assembly is filled with a charge of unmelted starting silicon material 105 which, upon heating, forms silicon material melt 106. As shown in FIG. 1, the charge is heated by the heater 104 until it is melted thoroughly. Each inner crucible 101, however, is subjected to deformation as shown in FIGS. 2 and 4, respectively, as the silicon material is melted.
In addition, since quartz has a high heat-insulating effect, the temperature is lower on the inner side of the inner crucible than on the outer side thereof. This may be advantageous during the growth of crystals since the silicon material introduced between the inner and outer crucibles can be melted efficiently, but it is disadvantageous when melting the silicon material initially fed in the crucible since the inner crucible itself prevents the heat generated by the heater from being transferred to its interior, so that the efficiency of the melting is lowered. If the quantity of heat applied to the material should be increased to reduce the time required for the melting, the inner crucible would be subjected to excessive deformation.
In Japanese Patent Application First Publication "Kokai" No. 55-47300, there has been proposed an apparatus as shown in FIG. 5. The apparatus comprises the inner crucible 101 fixedly secured to a fixed portion 108 through a support 107. Such an apparatus, however, has the disadvantages that the inner crucible 101 cannot be rotated together with the outer crucible 102. For growing a single crystal ingot, the crucible must be generally rotated at a speed of 5 rpm to 10 rpm. Without such rotation, the formation of single crystals may be difficult.
In conjunction with the use of the double crucible, Japanese Patent Application First Publication "Kokai" No. 58-204895 discloses an apparatus which comprises a mechanism for causing the inner crucible to rotate and move upwardly and downwardly. The apparatus, however, has the drawbacks that it is large and complex in construction, thereby being high in cost.